1. Field of the Invention
This invention relates to a semiconductor light emitting device, and more particularly to a light emitting device using nitride III-V compound semiconductors.
2. Description of the Related Art
Nitride III-V compound semiconductors represented by gallium nitride (GaN) (hereinafter called GaN semiconductors) has lately attracted much attention, with development of light emitting diodes (LED) using GaN semiconductors into practical use and realization of laser diodes using GaN semiconductors, and applications thereof as light sources of optical disc devices and others are expected.
GaN semiconductor lasers basically have a DH structure (double heterostructure) which sandwiches an active layer of GaInN between an n-type AlGaN cladding layer and a p-type AlGaN cladding layer. The n-type AlGaN cladding layer and the p-type AlGaN cladding layer have lower refractive indices and higher band gaps than the active layer, and function to confine light generated in the active layer and prevent overflow of carriers. GaN semiconductor lasers having a SCH structure (Separate Confinement Heterostructure) additionally include an n-type GaN optical guide layer between the active layer and the n-type AlGaN cladding layer, and a p-type GaN optical guide layer between the active layer and the p-type AlGaN cladding layer. Most of the GaN semiconductor lasers having heretofore attained continuous oscillation at room temperatures have this AlGaN/GaN/GaInN SCH structure. Among them, those having emission wavelengths in the band of 400 nm are so configured that the active layer is made of GaInN with the In composition of about 15%, and the n-type AlGaN cladding layer and the p-type AlGaN cladding layer are made of AlGaN with the Al composition of about 6 through 8%.
In these conventional GaN semiconductor lasers, operation currents and operation voltages are higher than those of AlGaAs semiconductor lasers and AlGaInP semiconductor lasers already brought into practice, and these are an issue for their practical use.
For reducing the operation current of a GaN semiconductor laser, it is effective to reduce the threshold current density. For realization thereof, there is a technique of decreasing the refractive index and increasing the band gap of the n-type AlGaN cladding layer and the p-type AlGaN cladding layer by increasing the Al composition thereof, to increase the optical confinement ratio xcex93 and prevent overflow of carriers. The p-type AlGaN cladding layer, however, exhibits the tendency that generation of carriers therein becomes difficult as the band gap increases. Therefore, if the Al composition of the p-type AlGaN cladding layer is increased to lower the threshold current density, it invites the problem that the resistance in the p-type AlGaN cladding layer increases and the operation voltage of the device increases.
FIG. 1 is a graph showing a relation between the Al composition of the p-type AlGaN cladding layer and the operation voltage in a conventional GaN semiconductor laser. Samples used for this measurement are of a ridge stripe type having the AlGaN/GaN/GaInN SCH structure, having the cavity length of 1 mm and the stripe width of 3.5 xcexcm. The operation voltage Vop in FIG. 1 is the value which was obtained by driving the samples with pulses at the room temperature under the conditions: frequency of 1 kHz, duty ratio of 0.5% and electric current of 100 mA. It is known from FIG. 1 that the operation voltage Vop rises as the Al composition of the p-type AlGaN cladding layer increases. Such rise of the operation voltage not only shorten the lifetime of the device but also prevents improvements of semiconductor lasers toward higher output powers. Therefore, there is a demand for development of GaN semiconductor lasers capable of oscillation with a low threshold current density while minimizing such increase of the operation voltage.
It is therefore an object of the invention to provide a semiconductor light emitting device using nitride III-V compound semiconductors capable of lowering the threshold current density with no substantial increase of the operation voltage.
According to the invention, there is provided a semiconductor light emitting device having a structure in which an active layer is sandwiched between an n-type cladding layer and a p-type cladding layer, the active layer, the n-type cladding layer and the p-type cladding layer being made of nitride III-V compound semiconductor, comprising:
at least a p-type AlGaN layer, an optical guide layer and a third p-type semiconductor layer, said third p-type semiconductor layer being a p-type cladding layer comprising at least a first and a second p-type semiconductor coating layer whereby the first p-type semiconductor coating layer nearer to said active layer being made of a semiconductor layer material having a larger band gap than that of the second p-type semiconductor coating layer, and the optical guide layer and the second p-type semiconductor coating layer being different in band gap from each other; wherein the optical guide layer has a smaller band gap than the first p-type semiconductor coating layer.
In the present invention, the nitride III-V compound semiconductors are composed of at least one kind of group III element selected from the group consisting of Ga, Al, In, B and Tl, and one or more group V elements which include at least N and may additionally include As or P.
In the present invention, the p-type cladding layer is made up of two or more semiconductor layers made of BxAlyGazIn1xe2x88x92xxe2x88x92yxe2x88x92zN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61 and 0xe2x89xa6x+y+zxe2x89xa61) and different in composition.
In the present invention, for the purpose of effectively preventing overflow of electrons into the p-type cladding layer, one of the semiconductor layers, which forms the portion near the boundary of the p-type cladding layer nearer to the active layer and has a larger band gap, is preferably located in a position within the distance of 100 nm from the boundary of the p-type cladding layer nearer to the active layer, and more preferably in a position within the distance of 50 nm from the boundary of the p-type cladding layer nearer to the active layer. Thickness of the semiconductor layer having the large band gap is preferably selected from the range from 20 nm to 100 nm in order to minimize the increase of the resistance by this layer.
In the present invention, the p-type cladding layer is typically composed of a first semiconductor layer nearer to the active layer and a second semiconductor layer on the first semiconductor layer because it is simple in structure but highly effective in preventing overflow of electrons, and the band gap of the first semiconductor layer is made larger than the band gap of the second semiconductor layer. In this case, the band gap of the first semiconductor layer is determined so that a barrier high enough to prevent overflow of electrons be made in the conduction band of the p-type cladding layer, for the purpose of ensure the function of this layer as a barrier layer. On the other hand, the band gap or the second semiconductor layer having a smaller band gap is determined to make the resistance as low as possible for the purpose of restricting the increase of the operation voltage while maintaining a certain value of refractive index which does not unacceptably deteriorate optical confinement in the vertical direction.
According to the semiconductor light emitting device having the above-summarized construction according to the invention, since the p-type cladding layer includes two semiconductor layers different in band gap from each other, and the part of the p-type cladding layer near the boundary with the active layer is made up of the semiconductor layer having a larger band gap than that of the other portion, and overflow of carriers (electrons), from the side of the n-type cladding layer to the side of the p-type cladding layer is therefore prevented. Therefore, increase of the resistance in the p-type cladding layer is minimized. As a result, it is possible to lower the threshold current density with no substantial increase of the operation voltage in the semiconductor light emitting device using nitride III-V compound semiconductors.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.